TURBINE OPERATION/ENERGY OUTPUT
What makes the rotor turn?
The rotor consists of the rotor blades and of the hub and is placed upwind of the tower and the nacelle on most modern wind turbines. This is primarily done because the air current behind the tower is very irregular (turbulent).
But, what maker the rotor turn? The answer seems obvious, the wind. But actually, it is a bit more complicated than just the air molecules hitting the front of the rotor blades. Modern wind turbines borrow technologies known from aeroplanes and helicopters, plus a few advanced tricks of their own, because wind turbines actually work in a very different environment with changing wind speeds and changing wind directions.
Have a look at the animation of the cut-off profile, cross section, of an airfoil of the wing of an aircraft. The reason why an aeroplane can fly is that the air sliding along the upper surface of the wing will move faster than on the lower surface. This means that the pressure will be lowest on the upper surface. This creates the lift, i.e. the force pulling upwards that enables the plane to fly.
The lift is perpendicular to the direction of the wind. The lift phenomenon has been well known for centuries to people who do roofing work: they know from experience that roof material on the lee side of the roof, that is the side not facing the wind is torn off quickly, if the roofing material is not properly attached to its substructure.
If an aircraft tilts backward in an attempt to climb higher into the sky quickly, then the lift of the wing will indeed increase, as the wing is tilted backwards, but as shown in the picture, all of a sudden the air flow on the upper surface stops sticking to the surface of the wing. Instead the air whirls around in an irregular vortex, a condition which is also known as turbulence. All of a sudden the lift from the low pressure on the upper surface of the wing disappears. This phenomenon is known as stall.
An aircraft wing will stall if the shape of the wing tapers off too quickly as the air moves along its general direction of motion. Of course, the wing itself does not change its shape, but the angle of the the wing in relation to the general direction of the airflow, known as the angle of attack, has been increased in the picture above. Notice that the turbulence is created on the back side of the wing in relation to the air current.
Stall can be provoked if the surface of the aircraft wing, or the wind turbine rotor blade, is not completely even and smooth. A dent in the wing or rotor blade, or a piece of self-adhesive tape can be enough to start the turbulence on the backside, even if the angle of attack is fairly small. Aircraft designers obviously try to avoid stall at all costs, since an aeroplane without the lift from its wings will fall like a rock.
Aircraft designers and rotor blade designers are not just concerned with lift and stall, however. They are also concerned with air resistance, in technical jargon of aerodynamics known as drag. Drag will normally increase if the area facing the direction of motion increases.
- Typical aerodynamic profile shapes: graphs with airfoil shapes, notes
- Profile (section) characteristics: parameters, airfoil geometry
- Typical profile characteristics (A): coefficients, dimensionless numbers
- Typical profile characteristics (B): parameters, (dynamic) stall
- Typical profile characteristics (C): attached and separated flow
- Typical profile characteristics (D): more notes, steady-unsteady
- The aerodynamics of the wind turbines